Shoot! No school or city. Those were actually going to be my first two suggestions. Don't worry though. I have a few more.
You could use a corporation. The nucleus would be the CEO's office. That would make his office walls the nuclear membrane. The mail room and delivery guys could be the endoplasmic reticulum. The manufacturing wing and shipping area could function as a combination of ribosomes and golgi complex. The solar panels on the roof could be chloroplasts or even mitochondria (or use on site generators). The hallways could represent the cytoplasm, and the building's exterior walls would be the cell membrane.
I have students do this same kind of "cell simile" project, and I've seen students have great success with having the cell be like a car. Engine is the mitochondria. Chip set is the nucleus, etc. I had a student a few years ago walk me through the analogy of how a cell was like a computer. One of my favorites was how a cell is like the military.